The invention relates to a plasma spraying device for spraying a coating onto a substrate, as well as to a method for introducing a liquid precursor into a plasma gas stream, and the use of such a plasma spraying device and/or such a plasma spraying method for coating a substrate.
The plasma torch is one of the most rugged, powerful and well-controlled plasma sources used in industrial technologies. In surface coating technology its principal application is in the field of thermal spray by injection of solid particles (Plasma Spaying).
A great variety of plasma spraying apparatuses for coating a surface of a workpiece with a spray powder are well known in the prior art, and are used widely in completely different technical fields. Known plasma spraying apparatuses often comprise a plasma spray gun, a high power direct-current source, a cooling aggregate and also a conveyer for conveying a substance to be sprayed into the plasma flame of the plasma spraying gun. Regarding classical powder spraying techniques, the substance to be sprayed is of course a spraying powder.
In atmospheric plasma spraying, an arc is triggered in a plasma torch between a water-cooled anode and a likewise water-cooled tungsten cathode. A process gas, usually argon, nitrogen or helium or a mixture of an inert gas with nitrogen or hydrogen, is converted into the plasma state in the arc and a plasma beam with a temperature of up to 20.000 K develops. Particle speeds of 200 to 800 m/s are achieved through the thermal expansion of the gases. The substance to be sprayed enters the plasma beam with the help of a conveyer gas either axially or radially inside or outside of the anode region.
New processes based on successful elements from the known plasma spray technology are currently more and more investigated in order to open new markets for advanced surface treatment. One of the routes is to use liquid or gaseous precursors (instead of solids) to allow thin film deposition by vaporizing and dissociating the precursors (Chemical Vapor Deposition, CVD).
US 2003/0077398 describes a method for using nanoparticle suspensions in conventional thermal spray deposition for the fabrication of nanostructured coatings. This method has the disadvantage that ultrasound must be used for dispersing the nanoparticles in a liquid medium before the injection into a plasma gas stream.
WO 2006/043006 discloses a method for coating a surface with nanoparticles as well as a device for carrying out this method, wherein the method is characterized in that it involves an injection of a colloidal sol of these nanoparticles into a plasma jet outside of the plasma torch.
U.S. Pat. No. 6,447,848 discloses a modified Metco 9 MB-plasma torch, wherein the powder injection port has been removed and replaced by a multiple injection nozzle for injecting different liquid precursors and slurries at the same time into the plasma flame. That is, the liquid precursor is also fed outside of the plasma torch into the plasma gas stream.
In particular, the injection of liquids in plasma jets is a complex task which notably differs from the injection of gas-carried solid particles as used in the above-described well-developed plasma powder spraying technologies. Therefore this involves specific developments by adapting the plasma torch operation parameters.
One major problem is that by the injection of liquids in a plasma nozzle of regular geometry known from the prior art, it is difficult to obtain a quasi-homogeneous distribution of the liquid and/or pressure in the plasma gas stream. The liquid cannot penetrate enough in the plasma gas stream and can freeze by the expansion on leaving a respective introducing duct through which the liquid is introduced into the plasma gas stream.
That is, the spontaneous vaporization of the liquid at low pressure and the consecutive release of the latent heat often lead to a freezing of the remaining fluid at the exit of the introducing duct using plasma spraying devices known from the prior art.
Another major problem is due to the supersonic nature of the plasma jet flow, with surrounding barrel shocks or compression waves which scatter the injected liquid jet or spray and hamper its penetration inside the jet core. This disqualifies the injection of liquids outside the plasma torch nozzle (under normal pressure) for most of the operating pressure foreseen for thermal plasma CVD (below 100 mbar).
On the other hand, the momentum of the injected liquid jet has to be high enough or the injection pipe should penetrate the plasma jet beyond the barrel shocks to avoid scattering. This requires either a high injection velocity, or results in excessive heat load onto the introducing duct. Due to all these limitations and complications, the injection of the liquid outside of the torch nozzle known from the prior art has turned out to be inappropriate to achieve a sufficient penetration of the liquid into the plasma gas stream.
However, an injection of the fluid inside the plasma torch has not been considered so far due to the difficulties arising from the design of known plasma spraying guns, in particular due to the complex cooling system including the water-cooled anode and cathode as mentioned above.